Why do people eat unhealthy, high-calorie fast food? Is it because they don’t realize how bad it is for them — or do they realize it and just don’t care?

Two years ago, New York City bet it was the former. The city government passed a law requiring all fast-food chains to display the calorie count beside their food listings, right next to the prices. The theory was that if you forced people to see how many calories they were consuming, they’d make healthier picks. But did it work?

Apparently so. A group of Stanford University professors got Starbucks — one of the chains that began listing calorie counts — to give them the records for 100 million transactions at stores in New York, Boston and Philadelphia, over a 14 month period. Boston and Philadelphia do not have a similar law in place, so they were the control group. Sure enough, when the scientists compared consumption patterns at Starbucks in the three cities, New York’s law appeared to have had a small but significant impact: People made lower-calorie orders. As their paper — “Calorie Posting in Chain Restaurants” (PDF here) — concludes:

We find that mandatory calorie posting does influence consumer behavior at Starbucks, causing average calories per transaction to decrease by 6% (from 247 to 232 calories per transaction). The effects are long lasting: the calorie reduction in NYC persists for the entire period of our data, which extends 10 months after the calorie posting commenced.

Here’s what really intrigued me, though: At Starbucks, people behaved differently with food than with beverages.

When they calorie postings went up, customers quickly switched to lower-calorie Starbucks foods. But they didn’t switch to lower-calorie beverages. The scientists found calorie intake from food dropped by 14%, while the drop in calories for beverages was “negligible.” Indeed, the majority of the overall calorie reduction — that 6% drop — consisted of customers simply deciding not to order any food while at Starbucks. But when it came to drinks? They barely changed at all.

This is rather surprising, because some of those Starbucks drinks are insanely high-calorie. Say you order a Venti “Caramel Brulee Creme” with nonfat milk? That’s 480 calories, 70 of which are fat. Or how about a Venti “Double Chocolaty Chip Frappucino Blended Creme” with whipped creme? Friend, you just inhaled a whopping 670 calories, 200 of which were pure fat.

Given how fattening these drinks are — and, frankly, how nauseating they are; I have no idea how anyone can gag back these syrupy cocktails — why wouldn’t customers, when presented with the calorie count, pick something healthier? Some of these things are like pouring rendered tallow directly down your throat. (I’m actually not kidding: If you ate a half stick of butter you still wouldn’t come close to the calorie count of Venti “Pumpkin Spice Frappucino Blended Creme.”) Who drinks these revolting concoctions? I mean, “Mint Chocolaty Chip Frappuccino blended creme with Whipped Cream”? Even the whipped cream has to be chocolate-flavored? These customers do realize that there’s probably a corner store only a block away selling actual, regular, normal coffee for like 50 cents, right?

Okay, I’ll calm down now. The question is, why did the calorie information change the way Starbucks customers ate — but not stuff they drank? The researchers don’t speculate. But I think it’s because of the psychology of Starbucks itself. People who go to Starbucks are motivated primarily by the desire for a drink, not a piece of food. So even if they’re forced to confront the calorie count on their favorite, hideously repellent bucket of candied snot (sorry, I can’t stop myself here), they’re probably not going to change their mind. They’ll just decide to forgo the chocolate graham crackers.

Indeed, if you want further proof of this, consider another fascinating bit of data the scientists discovered: Starbucks customers actually overestimate the amount of calories in their favorite drinks. Before the enforced nutritional-posting law came into effect, the scientists polled Starbucks customers and asked them to guess how many calories were in their beverages. Amazingly, fully three-quarters of customers overestimated the amount. They thought the drinks contained 90 calories more than in reality. So that helps further explain why the labeling didn’t shift the customers’ drinking habits: They already knew the drinks were bad for them, and they didn’t care. Heck, some of them were probably relieved to discover they were drinking a mere 680 calories, instead of 770. Woo!

I should point out that the New York Times ran a story today that suggests the calorie-labeling law may prove to be a dud in the long run. Apparently the trend line for Starbucks customers went in a U-shape: After the law came into effect, their calorie intake dipped significantly, but then began creeping up again, and holiday binge-eating brought it entirely back up in line with Philadephia and Boston. Still, I have to say I’m impressed that New York managed to get a 6% reduction at all.

Maybe they should just hire me to stand outside various Starbucks locations and hector people. Yeah, that’d work.

(The photo above of that totally gross Peppermint Mocha Twist drink comes via stephenccwu’s Creative-Commons-Licensed Flickr photostream!)